Signal generator

ABSTRACT

A key operated signal generator comprising a pair of interconnected transistor oscillators having twin-T notch-filter feedback networks in which one resistance in each filter is variable and consists of a plurality of resistive elements arranged in a series parallel configuration. The series parallel configuration is such that when a single button of the key is actuated, at least two resistive elements are connected in series in each filter and when two buttons are actuated simultaneously, at least one resistive element is connected in series with two or more parallel resistive elements in at least one filter.

United States Patent [15] 3,699,477 McKell 1 1 Oct. 17, 1972 [54] SIGNAL GENERATOR 3,424,870 1/1969 Breeden et a1 ..331/ 142 72 I t L M K ll W t L f tt l 1 men or f J c e es a aye 6 Primary Examiner-John Kominski v Attorney-R. J. Guenther et al. [73] Assignee: Bell Telephone Laboratories, incorporated, Murray Hill, NJ.

Filed: Sept. 7, 1971 Appl. No.: 178,126

US. Cl. ..331/142, 331/56, 331/179, 333/70 CR Int. Cl. ..H03b 5/26 Field of Search ..333/70 CR; 331/110, 142, 141, 331/140,179,56

[56] References Cited UNlTED STATES PATENTS 3,349,350 10/1967 Rittenbach ..331/179 [57] ABSTRACT ries with two or more parallel resistive elements in at least one filter.

10 Claims, 2 Drawing Figures PATENTEUUN n 1912 SHEET 1 [IF 2 PATENTEU 0m 1'! I972 7 saw a or 2 3% mo 35 Q3 35 moi a a a 35 3; o w w 5 STE a #2 3 0 o m 30% Q2 92 33a 35 30 m mo Q3 35 2; mm .93 u 3 m it m N w 3 mo mam 3; 3m 5 3m 3m 3m mo 3. 3K 3 NET m m 3 w 8; m w a m m N m 2: N M 2: 3.! 22 E1 8: 08 m8. STE a: $72 2 QTNI NI, E w 6E SIGNAL GENERATOR v FIELD OF THE INVENTION This invention relates to signal generators and within that field to oscillators that are selectively tuned by the operation of keying means associated therewith.

BACKGROUND OF THE INVENTION Signal generators of this type are now in widespread use in telephone dials referred to in the Bell System as TOUCH-TONE pushbutton dials. The signal generator in each of these dials produces a unique multifrequency calling signal in response to the actuation of each pushbutton. Each signal consists of one of a group of frequencies from a relatively low frequency band and one of a group of frequencies from a relatively high frequency band. The frequencies in each group, hereinafter referred to as the principal frequencies, are essentially uniformly separated from one another, and each dual frequency signal is indicative of an individual digit or other code symbol in accordance with a now standardized multifrequency code.

In the design of the TOUCH-TONE dial considerable thought was given to the arrangement, spacing, and shape of the buttons so as to provide fast, error free calling. However, due to the rapidity with which pushbutton dials can be operated, it was found that users tend to make more dialing errors with this type of dial than with a rotary dial, and the most common error is the simultaneous actuation of two buttons.

One approach to this problem is to have the signal generator transmit an invalid calling signal when two pushbuttons are simultaneously operated. An invalid calling signal, that is, a signal that differs sufficiently from a valid calling signal so as to be distinguishable by the central office receiver, is ignored. Therefore, if the user does not immediately recognize that he has operated two buttons simultaneously, the lack of response from'the central office after he has finished dialing causes him to redialthe telephone number. The user is slightly inconvenienced, but because dialing is fast and easy, little time is lost. A significant advantage of this solution is that it offers the potential of utilizing the double push signals for transmitting data other than that used for calling.

The signal generator disclosed in US. Pat. No. RE 25,507 issued to L. A. Meacham on Feb. 7, 1964, and now in common use in the TOUGH-TONE dial, has these features. It comprises a single transistor oscillator having an inductively coupled feedback network in cluding a pair of tank circuits that are selectively tuned to the principal frequencies by the operation of individual pushbuttons of the dial. When, however, two buttons are actuated simultaneously, a single, rather than a dual, principal frequency signal is generated. The single frequency signal is readily distinguishable from a dual frequency signal, but it is somewhat deficient insofar as serving as a data signal. This is because a single frequency signal is fairly susceptible to duplication by ordinary speech and background sounds. Thus,

there is the need to require either complete silence or a signal of extended duration for it to be effective as a data signal.

Another shortcoming of this dial is that because it utilizes inductive elements, it does not lend itself to integrated circuit technology. Integrated circuits provide increased reliability and performance and reduced cost, size, and weight. Therefore, effort has been directed toward designing a multifrequency signal generator that can be manufactured using these techniques and that also provides the above-described type of double push protection.

Such a signal generator is disclosed in US. Pat. No. 3,424,870 issued to R. L. Breedien and R. M. Rickert on Jan. 28, 1969. It comprises a pair of interconnected transistor oscillators each having a twin'T notch filter feedback network. One T of each notch filter consists of a pair of series resistances and a shunt capacitance while the other T consists of a pair of series capacitances and a shunt resistance. The values of the resistances and capacitances are all fixed except for one series resistance in each notch filter, and both of these series resistances are selectively varied by the operation of the pushbutton dial to tune the oscillators to the two principal frequencies associated with the operated button.

More specifically, each of the variable resistances consists of a plurality of parallel resistors, a single one of which is connected to the associated twin-T configuration responsive to the actuation of each individual pushbutton. When, however, two buttons are operated simultaneously, a pair of the resistors in one or both of the notch filters are connected in. parallel. One or both the oscillators are thereby tuned to generate a nonprincipal frequency, resulting in an invalid calling signal.

One drawback of this arrangement is that because the resistors are used independently of one another in the generation of valid calling signals, a large amount of resistance is required. In thin film circuitry, the greater resistance the greater the area that is needed on the substrate and therefore the fewer the circuits that can be placed on the substrate. Consequently from a cost standpoint, it is desirable to employ a circuit having a minimum total resistance.

Another drawback of this arrangement is that the double push signal is again deficient insofar as serving as a data signal. Although all of the double push signals are dual frequency signals, the dual frequencies of some of the signals are in essentially the same frequency band, and the frequenciesof one signal are so close that for all practical purposes they are indistinguishable. In addition, some of the other signals have one frequency outside of the choice frequency portion of typical voice band facilities.

SUMMARY OF THE INVENTION The signal generator of this invention is basically the same as the last-described signal generator and therefore it can be manufactured using integrated circuit techniques. However, it requires less resistance, and the double push signals are dual frequency signals that are readily distinguishable from valid calling signals and at the same time are readily usable as data signals.

The signal generator comprises a pair of interconnected transistor oscillators having twin-T notch filter feedback networks in which the variable resistance in each filter consists of a plurality of resistive elements arranged in a series parallel configuration. This series parallel configuration is such that when a single button is actuated, at least two resistive elements are connected in series in each filter and when two buttons are actuated simultaneously, at least one resistive element is connected in series with two or more parallel resistive elements in at least one filter.

This arrangement provides the latitude to select the resistive elements so that when a single button is actuated, the oscillators are tuned to generate a pair of principal frequencies and when any two buttons are actuated simultaneously, at least one oscillator is tuned to generate a non-principal frequency that is separated from a principal frequency by less than the uniform separation between principal frequencies. Optimally the resistive elements are selected so that when any two adjacent buttons are actuated simultaneously one or both oscillators are tuned to generate a non-principal frequency that is separated from a principal frequency by approximately half the uniform separation between principal frequencies.

DESCRIPTION OF THE DRAWING FIG. 1 is a simplified circuit diagram of a key-controlled multifrequency signal generator in accordance with the present invention; and

FIG. 2 is a table showing the 49 unique frequency combinations that can be generated by the signal generator of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, the key-controlled multifrequency signal generator of this invention, while not limited thereto, is employed with particular advantage as a telephone call transmitter, and, for purposes of convenience, it will be described in terms of the disclosure in the above-mentioned Breeden-Rickert patent, identical reference numbers being used where reliance is placed on that disclosure for particulars.

The signal generator includes a dial DL having a plurality of pushbuttons arranged in a rectangular array of rows and columns. The first three columns of pushbuttons carry the digits 1 through and the symbols and# and these pushbuttons, are now standard on all push button dials. The fourth column of pushbuttons carry the letters A through D, and while these pushbuttons are not standard on pushbutton dials, they are available when needed to provide additional signalling beyond that available with the standard dial.

The pushbuttons in each row actuate one of a group of normally open low frequency selecting switches Ll through L4, while the pushbuttons in each column actuate one of a group of normally open high frequency selecting switches 'Hl through H4. Thus, when any pushbutton is depressed, it actuates one low frequency selecting switch and one high frequency selecting switch, and the pair of switches actuated by each pushbutton is unique. In addition, each pushbutton when depressed actuates a common switch CS. A pushbutton dial of this type is disclosed in U. S. Pat. No. 3,479,470, issued to .l. H. Ham, Jr., on Nov. 18, 1969.

The switches L1 through L4 serve to tune a low frequency oscillator OL comprising a multistage transistor amplifier 501 and a twin-T notch filter circuit connected between an intermediate output stage and the input of the amplifier. The twin-T notch filter circuit includes a capacitance Cl and a capacitance C2 connected in series and shunted by a resistance R3 as the first T, and a resistance network RNl and a resistance R2 connected in series and shunted by a capacitance C3 as the second T.

Similarly, the switches H1 through H4 serve to tune a high frequency oscillator OI-I comprising a multistage transistor amplifier 601 and a twin-T notch filter feedback circuit. The notch filter circuit includes series capacitances C10 and C20 and a shunt resistance R30 as the first T, and a resistance network RN10 and a resistance R20 connected in series and a shunt capacitance C30 as the second T. I

The amplifiers 501 and 601 are described in detail in the Breeden-Rickert patent, and, as set forth therein, the amplitude of the output of each amplifier is limited by a circuit in shunt with a series capacitance in each feedback path, the shunt circuit comprising a blocking capacitor connected in series with a pair of oppositely poled parallel diodes. Furthermore, the output of the amplifiers 501 and 601 is connected to tip lead T of a telephone line by way of a path that includes terminal 701 and normally open common switch contacts CS and a bias network 801 bridged across tip and ring leads T and R provides a common source of biasing potential for the transistors of both amplifiers. A conventional voice network VN is also bridged across leads T and R, and the leads, respectively, include normally open switch hook contacts SH, and 8H that are closed when the telephone handset is removed from its cradle.

In twin-T notch filters, signals of a particular frequency are attenuated to a maximum degree while adjacent frequencies are attenuated to a lesser degree, resulting in a notch in the plot of the frequency spectrum. In addition, a l phase shift occurs for a signal oscillating at the notch frequency, while signals only slightly displaced from the notch frequency depart markedly from this phase shift. Consequently, in the feedback loop of an oscillator whose amplifier has shifted the phase by l80 from the input to the output, an additional l80 phase shift produces regenerative feedback at the notch frequency, and the sharp departure at the other frequencies serves to suppress the non-notch frequencies.

Notch filters areparticularly suited for tuning oscillators that generate only one narrow frequency band. To operate at several frequencies with equal selectivity, an oscillator requires a circuit that exhibits a common phase departure effect at each of the particular frequencies. This requirement is met in accordance with this invention by means of the resistance networks RNl and RN10 which, respectively, incorporate the frequency selecting switches L1 through L4 and H1 through H4.

Resistance network RNl comprises resistive elements R101, R102, and R103 connected in series, a re sistive element R104 connected to the juncture of re sistive element R101 and R102, a resistive element R105 connected to the juncture of resistive elements R102 and R103, and resistive elements R106 and R107 both connected to the end of resistive element R103 opposite to its juncture with resistive element R102. The resistive elements R104 through R107 are connected in parallel with one another, and each shunt path includes one of the normally open frequency selecting switches L1 through L4, each switch serving to connect selected resistive elements in the network into the feedback circuit of the low frequency oscillator DL.

The resistance network RN is the same as the resistance network RNl, comparable resistive elements being identified by the addition of one hundred to the reference numberQSuffice it to say that each of the normally open frequency selecting switches H1 through H4 is connected to an individual shunt resistive element in the networkRNlO and when actuated serves to connect selected ones of the resistive element R201 through 207 into the feedback circuit of the high frequency oscillator OH.

The principal frequencies withwhich the frequency selecting switches are respectively associated are standardized for telephone signalling purposes and are shown in the following table:

L1 697 Hertz H1 1209 Hertz L2 770 Hertz H2 1336 Hertz L3 852 Hertz H3 1477 Hertz L4 941 Hertz H4 1633 Hertz It is seen that these frequencies essentially lie within the 700 to 1,700 Hertz range which is the choice portion of the frequency band for typical voice band facilities. It is also seen that each of the frequencies associated with the frequency selecting switches L1 through L4 all lie within a relatively low frequency band and have a uniform separation of approximately 10 percent from one another. Similarly the frequencies associated with frequency selecting switches H1 through H4 all lie within a relatively high frequency band and also have a uniform separation of approximately 10 percent from one another.

The signal generator is permitted a deviation of i 1.5 percent from these principal frequencies, this being an economically feasible tolerance that can be maintained in a telephone set, and to allow for the reception of these frequencies by the central office receiver, a recognition bandwidth of approximately t 2.5 percent is provided there. Thus, signals deviating by less than 2.5 percent from the principal frequency fall within this recognition band and are accepted as valid signals, while signals deviating by more than 2.5 percent from the principal frequency are ignored as invalid signals.

The resistance networks RNl and RN10 of this invention permit the resistive elements to be selected so that when any single frequency selecting switch is closed, a frequency deviating by less than 1.5 percent from a principal frequency is generated, and when any wo switches are closed, a frequency deviating by more than 2.5 percent but less than 7.5 percent from a principal frequency is generated. This is possible because whenever one frequency selecting switch is closed, at least two resistive elements are connected in series, and whenever two frequency selecting switches are closed, at least one resistive element is connected in series with at least two resistive elements in parallel. This provides the latitude to select the values of resistive elements so as to tune theoscillators to thedesired frequencies.

Furthermore, the resistive elements can be selected so that when two adjacent frequency selecting switches are closed, the frequency generated deviates by approximately 5 percent from a principal frequency, or, in other words, is a half-step removed from a principal frequency. Such a frequency is capable of being used as a data signal, since it is separated far enough from the adjacent principal frequency so as to be recognized as a distinct signal by a receiver having a recognition band of i 2.5 percent for the principal and half-step frequencles.

As shown in FIG. 2, 49 distinct dual frequency signals can be generated by the signal generator of this invention when tuned to generate such half-step frequencies. The 33 data signals provided in addition to the sixteen calling signals are all generated by simultaneously depressing a pair of adjacent pushbuttons on the dial DL. When the depressed buttons are in the same row, the dual frequency signal comprises a principal low frequency and a half-step high frequency. Thus, for example, when the buttons 1 and 2 are simultaneously depressed, the switch L1 is closed in the resistance network RN1 to connect the resistive elements R101, R102, R103, and R107 in series and tune the low frequency oscillator OL to the principal frequency of 697 Hertz. In addition, the switches H1 and H2 are closed in the resistance network RN10 to connect the resistive elements R201, R202, and R203 in series with the resistive elements R206 and R207 connected in parallel, tuning the high frequency oscillator OH to the half-step frequency of 1,405 Hertz.

When the two simultaneously depressed buttons are in the same column, the dual frequency signal comprises a half-step low frequency and a principal high frequency. Thus, for example, when the buttons 6 and 9 are simultaneously depressed, the switches L2 and L3 are closed in the resistance network RNl to connect the resistive elements R101 and R102 in series with resistive elements R103 and R106 connected in parallel with resistive element R105, whereby the low frequency oscillator OL is tuned to generate the half-step frequency of 897 Hertz. In addition, the switch H3 is closed in the resistance network RN10 to connect resistive elements R201, R202, and R205 in series and tune the high frequency oscillator OH to the principal frequency of 1,477 Hertz.

Finally, when the two buttons are neither in the same column or the same row, the dual frequency signal comprises both a half-step low frequency and a halfstep high frequency. Thus, for example, when the buttons 7 and 0 are simultaneously depressed, the switches L3 and L4 are closed in the resistance network RNl to connect the resistive element R101 in series with the resistive elements R102 and R105 connected in parallel with the resistive element R104. The low frequency oscillator 0L is thereby tuned to generate the half-step frequency of 990 Hertz. Furthermore, the switches H1 and H2 are closed in the resistance network RN l0, tuning the high frequency oscillator OH to generate the half-step frequency of 1,405 Hertz in the same manner as described above.

In one specific embodiment these principal and halfstep frequencies are achieved with. the following values for the resistances and capacitances in the twin-T notch filter circuits:

R101, R201 54.6 Kilohms R102, R202 14.8 Kilohrns R103, R203 19.4 Kilohms R104, R204 26.0 Kilohms R105, R205 32.6 Kilohms R106, R206 39.6 Kilohms R107, R207 72.1 Kilohms R2, R20 16.8 Kilohms The total resistance in each notch filter circuit is 281.7 kilohms. The total resistance for each notch filter circuit in the Breeden-Rickert signal generator when it is tuned to generate the principal frequencies is 394.4 kilohms. Thus, it is seen that the signal generator of this invention provides both additional signalling capabilities and a sizable reduction in total resistance.

What is claimed is:

l. A signal generator comprising:

an oscillator;

a plurality of frequency selecting switches;

a frequency determining circuit;

the circuit being coupled to the oscillator by each of the switches to produce a different principal frequency, the principal frequencies having a preselected maximum separation; and

the circuit being coupled to the oscillator by any two of the switches to produce a frequency separated from the principal frequencies produced by either of the two switches, the separation from one of these two principal frequencies being less than the preselected maximum separation.

2. A signal generator as in claim 1 wherein the principal frequencies have an approximately uniform separation and the frequency determining circuit is coupled to the oscillator by any two frequency selecting switches to produce a frequency separated from one of the two principal frequencies by less than the uniform separation.

3. A signal generator as in claim 2 wherein the frequency determining circuit is coupled to the oscillator by'any two immediately adjacent frequency selecting switches to produce a frequency separated from one of the two principal frequencies by approximately half the uniform separation.

4. A signal generator as in claim 1 wherein the frequency determining circuit has only resistive and capacitive elements incorporated in a notch filter configuration, some of the resistive elements being arranged in a series parallel resistance network including the frequency selecting switches, the switches connecting selected ones of the resistive elements into the notch filter to provide frequency control.

5. A signal generator as in claim 1 wherein the frequency determining circuit comprises a twin-T notch filter, one of the T configurations including a pair of series capacitances and a shunt resistance and the other. T configuration including a pair of series re sistances and a shunt capacitance, one of the series resistances comprising a plurality of resistive elements arranged in a series parallel network including the frequency selecting switches, the switches connecting selected ones of the resistive elements to the other series resistance to provide frequency control.

6. A signal generator as in claim 5 wherein the resistive elements and the frequency selecting switches are arranged in the series parallel network so that when any single switch is actuated at least two resistive elements are connected in series, and when any two switches are actuated at least one resistive element is connected in series with at least two parallel resistive elements.

7. A signal generator as in claim 6 wherein the principal frequencies have approximately uniform separation and the resistive elements of the series parallel network are selected so that when any single switch is actuated the oscillator is tuned to generate one of the principal frequencies and when any two switches are actuated simultaneously the oscillator is tuned to generate a frequency that is separated from one of the two principal frequencies by less than the uniform separation.

8. A signal generator as in claim 7 wherein the resistive elements are selected so that when any two immediately adjacent switches are actuated, the oscillator is tuned to generate a frequency that is separated from one of the two principal frequencies by approximately half the uniform separation between principal frequencies.

9. A signal generator as in claim 5 wherein the series parallel network comprises three resistive elements connected in series, a first shunt resistive element connected to the junction of the first and second series resistive elements, a second shunt resistive element connected to the junction between the second and third resistive elements, and third and fourth shunt resistive elements connected to the other end of the third series resistive element, the frequency selecting switches respectively connecting each of the shunt resistive elements to the other series resistance.

10. A multifrequency signal generator comprising:

a first oscillator for generating frequencies within a relatively low frequency band, and a second oscillator for generating frequencies within a relatively high frequency band, each oscillator including;

an amplifier having an input point and an output point,

a feedback circuit connected between the output and the input point, the feedback circuit comprising a pair of T circuits connected in parallel, one T circuit consisting of a pair of series capacitances and a shunt resistance and the other T circuit consisting of a pair of series resistances and a shunt capacitance, one of the series resistances comprising a plurality of resistive elements arranged in a series parallel network; and

keying means including a plurality of pushbuttons, each pushbutton when actuated connecting at least two resistive elements in series in each series parallel network to tune the first oscillator to one of a plurality of principal frequencies within the low frequency band and tune the second oscillator to one of a plurality of principal frequencies within the high frequency band, the principal frequencies in each frequency band being approximately uniformly separated from one another, and each pair of pushbuttons when actuated connecting at least one resistive element in series with'two parallel resistive elements in at least one series parallel network to tune at least one oscillator to a frequency separated from a principal frequency by less than the uniform separation between principal frequencies.

Attest:

- McCOY M. Gibson, JR. 0. MARSHALL DANN Commissioner of Patents I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,699 Q 477 Dated October" 17 1972 Inventofl Ly' nn J. McKell It is certified that error appears in the above-identified patent and that said Letters Patentare hereby corrected as shown below: I

Column 7, line-2 change "Picofarads" to Microfaradls-;-;

line 3, change "Picofarads" to -Micr-'iofaradls'-.

Signed and sealed this 20th day bf August 1974.

sin I Attesting Officer FOR" PO-IOSO (1H,) 7 I USCOMNPDC 0 16 559 I ll. UOVIIIIIIIT PIIN'I'IIGOIHCI I. OBIS-SM. 

1. A signal generator comprising: an oscillator; a plurality of frequency selecting switches; a frequency determining circuit; the circuit being coupled to the oscillator by each of the switches to produce a different principal frequency, the principal frequencies having a preselected maximum separation; and the circuit being coupled to the oscillator by any two of the switches to produce a frequency separated from the principal frequencies produced by either of the two switches, the separation from one of these two principal frequencies being less than the preselected maximum separation.
 2. A signal generator as in claim 1 wherein the principal frequencies have an approximately uniform separation and the frequency determining circuit is coupled to the oscillator by any two frequency selecting switches to produce a frequency separated from one of the two principal frequencies by less than the uniform separation.
 3. A signal generator as in claim 2 wherein the frequency determining circuit is coupled to the oscillator by any two immediately adjacent frequency selecting switches to produce a frequency separated from one of the two principal frequencies by approximately half the uniform separation.
 4. A signal generator as in claim 1 wherein the frequency determining circuit has only resistive and capacitive elements incorporated in a notch filter configuration, some of the resistive elements being arranged in a series parallel resistance network including the frequency selecting switches, the switches connecting selected ones of the resistive elements into the notch filter to provide frequency control.
 5. A signal generator as in claim 1 wherein the frequency determining circuit comprises a twin-T notch filter, one of the T configurations including a pair of series capacitances and a shunt resistance and the other T configuration including a pair of series resistances and a shunt capaciTance, one of the series resistances comprising a plurality of resistive elements arranged in a series parallel network including the frequency selecting switches, the switches connecting selected ones of the resistive elements to the other series resistance to provide frequency control.
 6. A signal generator as in claim 5 wherein the resistive elements and the frequency selecting switches are arranged in the series parallel network so that when any single switch is actuated at least two resistive elements are connected in series, and when any two switches are actuated at least one resistive element is connected in series with at least two parallel resistive elements.
 7. A signal generator as in claim 6 wherein the principal frequencies have approximately uniform separation and the resistive elements of the series parallel network are selected so that when any single switch is actuated the oscillator is tuned to generate one of the principal frequencies and when any two switches are actuated simultaneously the oscillator is tuned to generate a frequency that is separated from one of the two principal frequencies by less than the uniform separation.
 8. A signal generator as in claim 7 wherein the resistive elements are selected so that when any two immediately adjacent switches are actuated, the oscillator is tuned to generate a frequency that is separated from one of the two principal frequencies by approximately half the uniform separation between principal frequencies.
 9. A signal generator as in claim 5 wherein the series parallel network comprises three resistive elements connected in series, a first shunt resistive element connected to the junction of the first and second series resistive elements, a second shunt resistive element connected to the junction between the second and third resistive elements, and third and fourth shunt resistive elements connected to the other end of the third series resistive element, the frequency selecting switches respectively connecting each of the shunt resistive elements to the other series resistance.
 10. A multifrequency signal generator comprising: a first oscillator for generating frequencies within a relatively low frequency band, and a second oscillator for generating frequencies within a relatively high frequency band, each oscillator including; an amplifier having an input point and an output point, a feedback circuit connected between the output and the input point, the feedback circuit comprising a pair of T circuits connected in parallel, one T circuit consisting of a pair of series capacitances and a shunt resistance and the other T circuit consisting of a pair of series resistances and a shunt capacitance, one of the series resistances comprising a plurality of resistive elements arranged in a series parallel network; and keying means including a plurality of pushbuttons, each pushbutton when actuated connecting at least two resistive elements in series in each series parallel network to tune the first oscillator to one of a plurality of principal frequencies within the low frequency band and tune the second oscillator to one of a plurality of principal frequencies within the high frequency band, the principal frequencies in each frequency band being approximately uniformly separated from one another, and each pair of pushbuttons when actuated connecting at least one resistive element in series with two parallel resistive elements in at least one series parallel network to tune at least one oscillator to a frequency separated from a principal frequency by less than the uniform separation between principal frequencies. 